1. Field of the Invention
The invention relates to a filament lamp for emission of yellow light. The invention relates especially to a filament lamp for emission of yellow light which is advantageously used for a head lamp or a fog lamp of a motor vehicle.
2. Description of Related Art
For example, a lamp which emits yellow light is used as the head lamp or fog lamp for a motor vehicle so that the driver acquires a suitable visual field even in thick fog or rain. In one such lamp, the standard range of chromaticity of the emitted yellow light is established by "ECE Regulation No. 37."
Conventionally, as the lamp for emitting yellow light which is used for a motor vehicle head lamp a lamp is known which comprises:
a filament lamp with a colorless, transparent bulb; PA1 a concave reflector which surrounds this filament lamp; and PA1 a front glass which is located in an opening of this concave reflector, and in which the front glass is colored yellow by a cadmium-based dye. PA1 a glass bulb, PA1 a filament which is located in the bulb; and PA1 a color film which is formed on the outer surface and/or the inner surface of the bulb, and is produced from a combined oxide of titanium, nickel and antimony.
However, since a heavy metal, such as cadmium or the like, has adverse effects on the human body, with respect to the environment, it is desirable not to use a material which contains a heavy metal, such as cadmium or the like, in a lamp for a motor vehicle.
As the means for producing a lamp which emits yellow light, providing the outside surface of the lamp bulb with a yellow colored film is known. A composition for producing a yellow colored film is known (published Japanese Patent Application HEI 6-92623). This composition is produced by a solution of an alkyl silicate, such as tetraethoxysilane or the like containing vanadium pentoxide powder with a grain size of less than or equal to 0.2 microns, finely distributed.
However, since a color film which is obtained by this composition does not satisfy the chromaticity range established by "ECE Regulation No. 37" or has a low light beam transmission factor, it is not suited for a color film for a lamp. Furthermore, when a dispersion mixing device, such as a ball mill or the like, is used to distribute the vanadium pentoxide powder, there is the danger that changes in the shade of the color film occur, and a reduction in the light beam transmission factor will arise, as a result of the fact that, when producing the composition, a powder is mixed in originating from the grinding medium and the inside wall of the device which has been formed by abrasion.
Furthermore, a filament lamp for emitting yellow light a lamp is known in which the outer surface of the bulb is provided with a multilayer film in which layers with a low index of refraction, such as silicon dioxide or the like, and layers with a high index of refraction, such a titanium dioxide or the like, are alternately placed on top of one another.
In one such multilayer film, light interference is used. Since it has the property that light is transmitted in a certain wavelength range, and at the same time, also has the property that light is reflected in a certain wavelength range, there is the disadvantage that, depending on the variation of the layer thickness and the radiation angle of the light, light besides yellow light is emitted.